This invention relates to optical waveguide devices and, in particular, to optical waveguide Bragg gratings.
Optical waveguide Bragg gratings are critical components in WDM communication systems. They perform several key applications including add/drop filtering, band filtering, and dispersion compensation. In these applications the grating is typically used as a reflective filter. Incident light within the stopband of the grating is strongly reflected whereas light outside the stopband is transmitted. An ideal Bragg grating would possess a rectangular amplitude filter function. The reflection would be unity within the stopband and negligible outside the stopband.
In practice, an important limitation on a realistic optical waveguide Bragg grating is cladding mode loss on the short wavelength side of the main reflection band. This short wavelength cladding mode loss is caused by grating-induced coupling from the core mode into backward propagating cladding modes. The cladding mode loss is seen in the transmission spectrum as sharp resonances on the short wavelength side of the Bragg resonance. The magnitude of the loss scales approximately with the square of the strength of the grating, and the loss is dramatically exacerbated when many gratings are cascaded. It thus imposes strict limitations on the design of optical networks that use grating-based technologies.
Proposed approaches to reduce cladding mode losses in optical waveguide Bragg gratings fall into two basic categories. The first is reduction of core-cladding coupling through special design of the core region. Such reduction can be achieved by the depressed cladding design, the high delta design and the photosensitive cladding design. The second basic category involves applying polymer surface coatings to smooth the sharp resonant structure of the cladding mode spectrum and achieve, instead, an approximately uniform background loss.
The depressed cladding design was proposed by Dong et al. in xe2x80x9cOptical fibers with depressed claddings for suppression of coupling into cladding modes in fiber Bragg gratingsxe2x80x9d, IEEE Photonic Technology Letters, Vol. 9, pp. 64-66 (1997). A conventional waveguide core is surrounded by a lighter doped cladding region i.e. a cladding with a lower index of refraction. The depressed cladding region suppresses the overlap of lower order cladding modes with the core. The transverse oscillations are stretched in the depressed cladding region, since the transverse resonance condition is associated with the optical path length (distance times refractive index). This approach has achieved moderate success. But the reduction is limited by the amount that the index can be reduced in the depressed cladding region.
The high delta design involves increasing the offset of the cladding mode from the Bragg resonance. This is achieved by increasing the effective core refractive index so that it is substantially above that of the lowest order cladding mode. The result is that the cladding mode resonances are offset from the Bragg resonance. Various groups have demonstrated that a waveguide with xcex94xcx9c2%, and a core diameter of dxcx9c2 xcexcm, results in an offset of xcx9c2-5 nm. Although the high delta principle has been demonstrated, the usable bandwidth is still limited by the onset of cladding mode loss. In addition there is a significant penalty incurred due to mode mismatch between the grating waveguide and the transmission waveguide.
The photosensitive cladding design incorporates photosensitive material into the cladding. See E. Delevaque et al. xe2x80x9cOptical fiber design for strong gratings photoimprinting with radiation mode suppression,xe2x80x9d OFC ""95, PD5, (1995) and K. Oh et al., xe2x80x9cSuppression of cladding mode coupling in Bragg grating using GeO2B2O3 doped photosensitive cladding optical fiber,xe2x80x9d Electronic Letters, Vol. 35, pp. 423-424 (1999). After UV exposure, the grating region extends into the cladding. If the core and cladding have the same sensitivity and there is no blaze, and the exposure is uniform through the waveguide, then the grating will have negligible coupling to cladding modes. Thus cladding mode loss will be negligible. A disadvantage of this scheme is a net reduction in the grating strength due to absorption in the photosensitive cladding region. There is also increased coupling to asymmetric modes because of the increased asymmetry in the region where these modes have a large mode field strength.
Turning to the second basic approach, the waveguide is typically surrounded with a lossy polymer material that has a refractive index near that of the cladding glass. In this case the cladding mode extends into the polymer where it is absorbed, and thus core-cladding mode coupling is reduced. The cladding mode loss is reduced closer to the radiation limit, typically by a factor of 4-5. This loss is acceptable for many applications but can still limit the number of devices that can be cascaded. Accordingly, there is a need for improved optical waveguide gratings having reduced cladding mode loss.
In accordance with the invention, an optical waveguide comprising a longitudinally extending core housing an optical grating and a cladding layer peripherally surrounding the core, is provided with an outer surface of the cladding layer having perturbations. Each perturbation has a height with respect to the core that varies by at least 0.1 times a Bragg wavelength of the grating over the surface of the perturbation and covers an extent of the outer surface whose linear dimensions are less than 1 cm. The perturbations suppress cladding mode spectra and reduce short wavelength cladding mode loss.